A variety of new techniques have recently been developed for the study of atomic scale structure of solids. In addition to conventional diffraction, several new methods of utilizing X-ray radiation to obtain structural information have recently been explored. The fine structure observed near the X-ray absorption edge of an atom in a solid is the basis for one such technique commonly referred to by the acronym EXAFS (Extended X-ray Absorption Fine Structure). This technique gives detailed information concerning the local environment of a specific atomic species.
Recent studies and theoretical calculation suggest that a corresponding fine structure should occur when an X-ray is Compton scattered by a bound electron which is subsequently excited to a higher unoccupied energy level. This scattering can be called either Raman or modified Compton scattering (MCS). By carrying out an energy dispersive analysis of X-rays so scattered, it should be possible to obtain information identical to that obtained by EXAFS. The MCS technique would be ideally suited for studying light elements where EXAFS experiments are generally difficult to carry out.
Another type of X-ray experiment involves the use of anomalous X-ray dispersion to extend conventional diffraction studies to multicomponent materials. By using this effect, one can combine the information obtained in two experiments utilizing different characteristic X-ray radiations to derive the atomic environment of each separate component in the material.
Scientists are particularly interested in understanding the properties of amorphous (noncrystalline) materials. Both of the techniques described above would provide precisely the type of structural information required to understand atomic arrangements in such materials. This information in turn can be used to understand the macroscopic properties of noncrystalline solids.
X-ray and neutron diffraction techniques have always offered the most straightforward method of studying the structure of materials. For multicomponent alloys, however, specific quantitative information on the individual atomic species is in general very difficult to extract from these experiments for a number of reasons. A strong effort is therefore being made by many researchers to take advantage of several relatively new techniques, such as EXAFS, anomalous dispersion studies, and combined neutron and X-ray diffraction studies, to obtain much more specific statistical structural information. What is desired for this effort is a high resolution, high acquisition rate, X-ray instrument which can be used in several different configurations.
The typical arrangement of apparatus used in a study of Compton scattered X-rays utilizes two flat crystals. Experiments using a double-crystal spectrometer have been reported in the literature. K. Das Gupta, Phys. Rev. Lett., 21, 338 (1964) is selected as representative because it best illustrates the geometry, and the problem of making fine coordinated adjustments in the angular positions of the crystals. A more complex triple-crystal geometry was used in an experiment reported by N. G. Alexandropoulos and G. G. Cohen, Phys. Rev., 187, 455 (1969). A single spectrometer arrangement has also been used with two slit tubes in an experiment reported by K. Das Gupta, Phys. Rev., 128, 2181 (1962). It required three coordinated adjustments: (1) the sample orientation with respect to the X-ray beam, (2) the position of a slit tube in the path of the sample reflected beam so that the mean scattering angle .phi..sub.m is known, and (3) the angular position of a second slit tube so that the rays scattered at .phi..sub.m make the proper Bragg angle through a quartz crystal. A second, more complex single-crystal method is also illustrated by K. Das Gupta. These two methods are variants of the double-crystal spectrometer arrangement which has been in use since at least 1930. See J. A. Bearden, Phys. Rev., 36, 791 (1930), N. S. Gingrich, Phys. Rev., 36, 1050 (1930), and W. M. DuMond and H. A. Kirkpatrick, Phys. Rev., 37, 136 (1931).
What is needed is a high resolution energy analyzer requiring only one manual adjustment to quickly carry out experiments to measure modified Compton scattering in solids and to study the X-ray absorption edge and the associated extended X-ray absorption fine structure (EXAFS). From the Compton scattering data, it is possible to extract information similar but complementary to that available from conventional EXAFS experiments. To study the X-ray absorption edge and the associated EXAFS, A. A. Bahgat and K. Das Gupta devised a single-crystal geometry as reported in Rev. Sci Instrum, 50, 1020 (1979). A microfocus X-ray tube with a small focal spot is positioned on the circumference of a circle centered on the single Si crystal such that the entire crystal is exposed to the X-ray beam passing through a sample. A film or moving detector on an arc (segment of a circle centered on the virtual image point of the crystal reflected energy) yields the absorption spectrum of the sample. However, this relatively simple arrangement lacks the desired versatility of an instrument capable of also carrying out Raman or modified Compton scattering experiments, and also requires the use of some type of position sensitive detector or a scanning mechanism.
A major limitation inherent in EXAFS studies has been sufficient intensity or radiation flux. As reported by P. Eisenberger and B. M. Kincaid, Science, 200, 1441 (1978), the increase in flux of approximately 10.sup.5 to 10.sup.6 provided by the Stanford synchrotron has vastly expanded the use of EXAFS in the study of materials. The authors summarize the current quantitative understanding of EXAFS, and describe how measurements are made using the Stanford synchrotron. Unfortunately, the availability of the Stanford synchrotron for EXAFS studies of materials is limited. More EXAFS studies could be carried out if there were available an instrument of high flux intensity that could be placed in the laboratory of virtually every scientist interested in carrying out EXAFS experiments as and when needed. At least one such laboratory instrument has been developed, as reported by G. S. Knapp, H. Chen and T. E. Klippert, Rev. Sci. Instrum., 49, 1658 (1978). It uses a Johanssen cut Germanium crystal as a focusing monochromator to increase flux intensity. Both the radiation source and the detector are positioned outside the Rowland circle of the Johanssen cut crystal. The sample is then placed in the beam between the crystal and the detector. The arrangement was implemented using a standard General Electric XRD-5 horizontal diffractometer, with the result that a complex alignment procedure is required. The procedure outlined requires four physical elements to be aligned. Although complete alignment procedures are required only for initially setting up an experiment, many adjustments are still required for each different measurement over the entire spectrum, and a microprocessor is necessary to drive the three independent positioning motors which are required.